Isolated systems, such as AC/DC converters or DC/DC converters (which require isolation ratings of greater than 1,500V) have long utilized a combination of an error amplifier/reference and an optocoupler to feedback output voltage information and to close the loop of the isolated system. The most commonly used device for creating an isolated system is a TL431 shunt reference (an error amplifier and optocoupler combination) and a standard optocoupler. There are many companies which make variants of the TL431 shunt reference and the TL431 is one of the highest volume semiconductor devices in the industry.
Over time, improvements on the TL431 have been made including low power CMOS implementations and versions with wider bandwidth capabilities. Similar improvements have also been made to other optocoupler components. Such solutions are known as wide bandwidth isolators. Products including wide bandwidth optical isolators are used for measuring current in isolated AC/DC or DC/AC systems such as solar panels or motor drivers. These high bandwidth systems provide critical protection information and therefore any reliability deficiency is of serious concern. By communicating only a subset of a sigma delta modulators quantizer across the isolator but closing the modulator loop on the primary side, much wider bandwidth communications of analog information can be achieved than with optocouplers. This allows for the use of the proposed isolator for cycle by cycle loop control and protection functions which previously required components on the local side of the isolation.
Regardless of the improvements, optoisolators have a tendency to frequently wear out, and are considered one of the least reliable components in an isolated system. To overcome the lack of operational robustness and reliability of optoisolators, various solutions have been introduced which range from using multiple optocouplers for redundancy, to primary side feedback controllers which attempt to forgo an optocoupler altogether. Although primary side feedback and redundancy have been adopted in various applications, redundancy is costly and still does not eliminate the wear out mechanisms in optocouplers. Further, serial redundant systems are even more expensive due to the components which have to determine when to move to a different or newer optocoupler. Parallel redundant systems improve statistical failures but the tendency of the optocoupler to wear out remains. Primary side feedback has eliminated optocoupler components in various systems but at the price of accuracy, complexity, and periods during which output information is unavailable.
Additional methods of creating isolation include near field RP isolators as well as magnetically or capacitively coupled systems. However, RF isolators tend to be expensive and use a lot of power, as RF isolators require dual die and operate at very high frequencies, and while magnetically and capacitively coupled monolithic systems are good monolithic solutions, they are AC only systems and are generally suitable for communicating digital data only.
Z-domain techniques, which utilize delay cells and discrete mathematics to create transfer functions have long been recognized as an efficient replacement for various continuous time systems. Discrete systems, however, can result in non-desirable limit cycle oscillations when used to control feedback loops. The spectrum of these oscillations can be difficult to predict, and therefore transfer functions created from delay or digital techniques are rare in AC/DC and DC/DC isolated systems. While limit cycle oscillations can be rendered too small to be relevant as well as predictable by increasing the effective number of bits of quantization, this requires high order data converters which are too slow for AC/DC and DC/DC applications.
It would therefore be desirable to create an isolated system capable of utilizing capacitive coupling which could be used to replace the functionality of an optically isolated feedback system, while eliminating existing isolation method deficiencies and despite the AC only restrictions of capacitive coupling and further to eliminate concerns about limit cycle oscillations.